Pulse duration lengthener



Feb. 4, 1958 w. R. AIKEN 2,822, 13

PULSE DURATION LENGTHENER Filed July 27, 1953 POWER SUPPLY I I l 1 b2 INVEN TOR.

. WILL/AM Ross AIKEN fl /d /QM A T TORNE Y United States Patent 2,822,473 PULSE DURATION LENGTHENER William R. Aiken, Berkeley, Calif., assig'ndr to the United States of America as represented by the United States Atomic Energy Commission Application July 27, 1953, serial No. 387,554

4 Claims. (Cl. 250=-27) The present invention relates to a method and apparatus for changing the frequency bandwidth of pulsed signals and more particularly for decreasing the band width of such signaIs.-

It is well'known that pulses comprise a large number of ditferentfrequency components and that the number increases as the pulse rise time decreases. Thus, for pulses having a very short rise time, it is necessary to build special and complex equipment to handle a wide band of frequencies and such statement is particularly applicable with respect to transmission and reproduction apparatus where the standard of quality of results is high. It is also well known that the number of higher frequency components is increased as the pulse duration is decreased. To extend the use of available commercial apparatus havingsubs'tantially low frequency and narrow bandwidth ice cathode 13, through the control grid 14, and through the aperture of the accelerating electrode 16 along the axis of the envelope 11. Mounted within the envelope 11 along the axis thereof is an elongated and hollow cylinder 17 of conducting material which serves as a drift tube for the beam of electrons: A collector or anode 18 is disposed at the other end of the envelope 11 to receive electrons which pass through the central opening of a shield disc 19 suitably mounted between the drift tube 17 and ,the anode. Each ofthe elements, described above as being mounted Within the envelope 11, is suitably sup; ported therein and provided with necessary electrical leadsextending through the wall of the envelope in a vacuum-tightmanner. p

To supply suitable operating potentials for the device there is provided acpnventionalpower supply 21 having a voltage divider 22- connected between the terminals characteristics to the high frequency and wide band region, 1

now"- limited to special apparatus, the present" invention expands the risetime and time base of the signal pulse Suchexpansion is achieved by amplitude modulationof an electron beam with'the signal pulse and directing the resultant beam of electrons into a drift tube having a decreasing applied voltage. The result of the foregoing is to progressively slow down the modulated beam so that an expanded pulse having the characteristics of the original pulse emerges from the drift tube to be collected and impressed upon transmission or reproduction apparatus.

It is therefore an object of the present invention to provide a method and apparatus for changing. the frequency bandwidth of pulsed signals.

Another object of the invention is to provide a rise time and timeb-ase expander for signal pulses having a veryshort duration.

A further object of the invention is to provide a device which effectively expands the useful frequency range of apparatus having low frequency and wideband characteristics.

A still further object of the invention is toprovide a device wherein signal pulses having a short rise time and duration are progressively delayed to form signal pulses having a longer rise time and duration with other characteristics remaining substantially the same.

Otherobjects and advantages of the present invention will be apparent in the following description and claims considered together with the accompanying drawing, in which:

Figure l is a schematic wiring diagram of the invention; and

Figure 2 is a series of time correlated voltage waveforms occurring during operation of the invention of Fig. 1.

Referringto the drawing in detail, Fig. 1 in particular, there is provided an elongated and evacuated envelope 11. Disposed within one end of the envelope 11 are elements of an. electron gun, 12 comprising a cathode 13, control grid 14, and apertured accelerating electrode 16. Such elements "of the electron gun 12 are mounted axially of thetube so that an electron beam progresses from the thereof.- Such voltage divider 22 comprises three resistive sections 23, 24, and 26 with the section 23 con= nected at one end to the negative terminal of the power supply .21 and being-variable. A connection is made from thecathode 13 to the junction between two of the resistive sections 23 and 24*which is grounded. A resistor 27 is connected between the control grid 14 and the adjustable element of the variable resistive section 23 which is provided with a by-pass capacitor 28 connected to ground. The accelerating electrode 16 is directly con nected to the positiveterminal of the power supply 21 and the anode 18 is connected to the other junction be= tween the two resistive sections 24 and 26 through a droppingresistor 29. u V

-A signal input terminal 31 is providedand connected to the control grid 14 through a coupling-capacitor 32 to amplitude.modulate'the beam of electrons emerging from the electron gun 12. The input of a pulse generator 33 is connected to the signal input terminal 31 so that the generator is initiated to generate an output voltage in synchronism with the input signal pulse; The pulse gen= erator 33 is conventional arid has an output voltage which is exponential in form. A direct connection is -made from the output of the pulse generator- 33 to the control grid ofa triode type driver tube 34. The cathode of such tube 34 is connected to the positive terminal of a conventional power supply 36, the negative terminalof which is connected to ground. The anode of the tube 34 is directly connected to the drift tube 17 and a resistor 38-is connected between the'drift tube and the'juncfior'i between the two resistive sections 24 and 2-6 of the volt-age divider 22. To complete-the connectionsthe shield disc 19 is directly connected to'ground and an output terminal 39 is coupled to the anode 18-by a coupling capacitor 41 connected therebetween;

From the foregoing description it is apparent that the cathode 13 is held at ground potential, that the control grid 14 is biased to normally prevent electrons from passing therethrough, thatthe accelerating'elect-rode 16 is-irnpressed with a positive voltage with respect to'the cathode, that the drift tube 17 and anode 18"are impressed with a voltage intermediate in valuebetween ground and that of the accelerating electrode, and that the-shield disc 19 is grounded. Under such conditions theoceurrence of a positive pulse 51 (as shown in Fig; 2) at the input terminal 31 results in a corresponding pulse of electrons being accelerated toward the drift tube17. The input pulse 51 at the terminal31 is also impressed upon the input of the pulse generator 33 to initiate operationthereof to develop. an exponentially increasing voltage 52 (see Fig. 2) which is impressed upon the control grid of the tube 34. The increasing conductivity of the tube 34 causes a decreasing voltage 53(see Fig,- 2) to develop at the anode of thetube which is directly connected to the drift tube 17. Thus at the time the pulse of electrons e2 is starting to enter the drift tube a maximum value of voltage is applied. Since the voltage impressed upon the drift tube 17 decreases during the duration of the pulse, the electrons are subject to a decreasingtelectric field between the accelerating electrode 16 and the drift tube. The result of such action is that the electrons of the pulse of electrons are progressively slowed down depending upon the value of voltage on the drift tube 1'7 at the time of entry. From the foregoing it'follows that the transit time of the electrons is directly proportional to the length of the drift tube 17 and inversely proportional to the velocity of the electrons.

Since the input at the terminal 31 is a pulse which develops a pulse of electrons flowing through the accelerating electrode 16, the spread or duration of the output pulse is determined by the difference between the transit times of the electrons at the minimum and maximum values of voltage applied to the drift tube 17. The expanded pulse of electrons is collected at the anode 18 and the resulting current flow through the resistor 29 causes a decrease in anode voltage which is coupled to the output terminal 39 as the output voltage 54 (see Fig. 2).

In the foregoing it has merely been stated that the electrons are progressively slowed down by the decreasing voltage of the drift tube 17 and analysis of the action reveals that there is a quadratic relationship between the velocity and voltage. To substantially compensate for such relationship and achieve time linearity within the invention, the voltage 53 of the drift tube 17 is selected to be exponential, as previously described. By selecting an exponential voltage for the drift tube 17 it is not intended to limit the invention thereto because a voltage varying according to the stated quadratic relationship between the velocity and voltage would give absolute time linearity. Such voltage is difficult to develop and the degree of improvement is slight so, for simplicity of illustration and description, the exponential voltage has been selected.

With a foot length of drift tube having an impressed decreasing voltage range of 500 volts to 100 volts, a 0.10 microsecond pulse is expanded to an output pulse of 0.38 microsecond duration and a 0.05 microsecond pulse is expanded to an output pulse of 0.33 microsecond duration. It will be readily apparent that the above-stated figures of expansion will be different for other lengths of drift tube 17 and, also, for other ranges of voltage. With the examples given for a 10 foot length of drift tube 17 the power supply 21 is set to deliver a voltage from -50 to +3000 volts between the two terminals and the voltage divider 22 arranged so that a +500 volt potential difference exists across the second resistive section 24. Also the power supply 36 in the cathode of the triode tube 34 is selected so that a suitable operating voltage exists between the anode and cathode.

The circuit comprising the pulse generator 33, driver tube 34, and power supply 36 merely illustrate one manner of accomplishing the desired drift tube 17 voltage. Another type circuit, which would serve the purpose of such elements, is a differentiator type circuit connected to the terminal 31 serving to trigger a gaseous discharge tube and discharge the capacitance of the drift tube 17 to ground through a predetermined value of resistance.

It is Well known that the velocity of a particle is inversely proportional to the square root of the mass of the particle and from such fact it is apparent that heavy particles may be utilized in the place of electrons. Under such circumstance the length of the drift tube 17 may be shortened by a large factor over the required length with electrons for a given expansion factor.

The invention has been described with respect to the particular use as a frequency bandwidth expander for pulses; however, it is readily apparent that the invention may be utilized as a frequency changer and for other applications, For certain applications time-linearity 4 between input and output is not essential and other means of drift tube 17 bias may be used.

While the salient features of the present invention have been described in detail with respect to one embodiment it will be apparent that numerous modifications may be made within the spirit and scope of the invention and it is therefore not desired to limit the invention to the exact details shown and described except insofar as they may be defined in the following claims.

What is claimed is:

1. A frequency bandwidth changer comprising an evacuated envelope, means disposed in said envelope for establishing a beam of charged particles, grid means disposed in the path of said beam, terminal means coupled to said grid means for amplitude modulating said beam in response to a signal having a wide band of frequencies at said terminal means, a single drift tube disposed in said envelope coaxially with said beam, a voltage generator connected to said terminal means responsive to said signal to develop a voltage varying in amplitude in the same sense for the duration of said signal, means connected between said drift tube and said voltage generator for applying said voltage, and means disposed in said envelope for collecting said beam of particles at the exit of said drift tube, the length of said drift tube and the maximum and minimum values of said voltage proportionately correlated with an expansion time, whereby the width of the band of frequencies of said signal is substantially altered at said collecting means.

2. In a frequency bandwidth expander, the combination comprising an evacuated envelope, means disposed in said envelope for establishing a beam of electrons, grid means disposed in said envelope in the path of said beam, terminal means coupled to said grid means for amplitude modulating said beam in response to a signal having a wide band of frequencies at said terminal means, a single drift tube disposed in said envelope coaxially with said beam, a voltage generator connected to said terminal means responsive to said signal to develop a voltage varying continuously in amplitude from a minimum to a maximum value during the time of said signal, inverting means connected between said generator and said drift tube, and means disposed in said envelope for collecting said beam of electrons at the exit of said drift tube, the length of said drift tube and the maximum and minimum values of said voltage proportionately correlated with an expansion time, whereby the width of the band of frequencies of said signal is substantially linearly expanded at said collecting means.

3. In a frequency bandwidth expander, the combination comprising an evacuated envelope, means including a control element for establishing a beam of charged particles in said envelope, coupling means connected between said control element and an input of a signal having a wide band of frequencies, a single elongated conducting cylinder disposed in said envelope coaxially with the path of said beam, means connected between said input of a signal and said cylinder for impressing an exponentially varying voltage in response to and for the duration of said signal, said voltage varying in amplitude with the same polarity as that of the charge of said particles, and means disposed in said envelope for collecting said beam of particles at the exit of said cylinder, the length of said cylinder and the maximum and minimum values of said voltage proportionately correlated with an expansion time, whereby the width of the band of frequencies of said signal is substantially linearly expanded at said collecting means.

4. In a frequency bandwidth changer, the combination comprising an evacuated envelope, means including a control element for establishing a beam of charged particles in said envelope, an input terminal for connection to a source of a signal having a wide band of frequencies coupled to said control element, a single elongated con ducting cylinder disposed in said envelope coaxially with the path of said beam, a pulse generator of an expo- 5 flentially varying voltage having an input and an output with the input connected to said input terminal, a vacuum tube having at least a cathode, control grid, and anode, said control grid connected to the output of said pulse generator, a bias power supply connected between said cathode and ground, a dropping resistor connected between said anode and a source of positive operating potential, a connection extended between said anode and said cylinder, and means disposed in said envelope at one end of said cylinder for collecting said particles, the length of said cylinder and the maximum and minimum values of said exponentially varying voltage at said cylinder proportionately correlated with an expansion time, whereby the width of the band of frequencies of said signal is substantially linearly changed at said means for collecting said particles.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,506 Hahn June 27, 1944 2,147,454 Morton Feb. 14, 1939 2,239,678 Jobst Apr. 29, 1941 2,288,256 Shepherd June 30, 1942 2,500,945 Hansen Mar. 21, 1950 2,516,886 Labin Aug. 1, 1950 2,543,082 Webster Feb. 27, 1951 

